Surface light source device and display apparatus

ABSTRACT

A surface light source device includes substrate, a plurality of light emitting devices disposed at constant intervals on substrate, and light diffusion plate disposed substantially parallel to substrate over the plurality of light emitting devices. Light emitting device includes light emitting element and light flux controlling member. A luminous intensity of light from light emitting device is gradually increased as an angle relative to optical axis becomes larger in an angular range from a direction along optical axis of light emitting device to a direction of emission of light with the highest luminous intensity from light emitting device. An angle of the light with the highest luminous intensity from the light emitting device is more than 78.7° relative to the optical axis. Surface light source device satisfies two Equations of H/P≦0.2, and I 1/2 /I 0 &gt;6.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/604,984, filed on Jan. 26, 2015, which claims the benefit of JapanesePatent Application No. 2014-013149, filed on Jan. 28, 2014, and JapanesePatent Application No. 2014-128061, filed on Jun. 23, 2014, thedisclosures of which including the specifications, drawings andabstracts are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a surface light source device and adisplay apparatus having the surface light source device.

BACKGROUND OF THE INVENTION

Some transmission type image display apparatuses such as liquid crystaldisplay apparatuses and signboards use a direct surface light sourcedevice as a backlight. In recent years, direct surface light sourcedevices having a plurality of light emitting elements as the lightsource have been used. In such a surface light source device, there isdisposed, over a light emitting element, a light flux controlling memberfor controlling the distribution of emission light from the lightemitting element (see, for example, PTL 1).

FIGS. 1A and 1B are drawings illustrating the configuration of surfacelight source device (surface light emitting unit) 10 set forth in PTL 1(see FIGS. 8A and 8B of PTL 1). FIG. 1A is a perspective view of surfacelight source device 10, and FIG. 1B is a sectional view of surface lightsource device 10. As illustrated in these drawings, surface light sourcedevice 10 has housing 20, substrate (mounting plate) 30 disposed insidethe housing, and a plurality of light emitting devices (light emittingunits) 40 disposed at constant intervals on substrate 30. Light emittingdevice 40 has a light emitting element 42, and a light flux controllingmember (lens part) 44 disposed over light emitting element 42. Anincidence surface and an emission surface of light flux controllingmember 44 are formed so as to expand the emission light from lightemitting element 42. Here, it is assumed that light diffusion plate 50is disposed at an opening of housing 20. The upper surface of lightdiffusion plate 50 functions as a light emitting surface.

FIG. 2 is a graph illustrating the light distribution characteristics oflight emitting device 40 (see FIG. 6 of PTL 1). As illustrated in thisgraph, a large amount of light is emitted from light flux controllingmember 44 in a larger angular range relative to the optical axis oflight emitting device 40, whereas almost no light is emitted from lightflux controlling member 44 in a smaller angular range relative to theoptical axis of light emitting device 40. Therefore, when only one lightemitting device 40 is disposed on substrate 30, the luminance of an areaof the light emitting surface directly above light emitting device 40 isdecreased. However, when a plurality of light emitting devices 40 aredisposed on substrate 30, emission light from one light emitting device40 reaches an area directly above the other light emitting devices 40,as illustrated in FIG. 1B. As a result, the luminance unevenness on thelight emitting surface becomes smaller in surface light source device10.

As illustrated in FIG. 1B, the center-to-center distance (pitch) betweenlight emitting devices 40 is set as P (mm), and the interval (height)between the upper surface of substrate 30 and the lower surface of lightdiffusion plate 50 is set as H (mm). Referring to FIG. 8B of PTL 1 (FIG.1B of the present application), in surface light source device 10 setforth in PTL 1, H/P is about 0.77.

CITATION LIST Patent Literature PTL 1 Japanese Patent ApplicationLaid-Open No. 2009-152142 SUMMARY OF INVENTION Technical Problem

As illustrated in FIG. 2, in order to eliminate the luminance unevennesson a light emitting surface, surface light source device 10 set forth inPTL 1 is adjusted such that the peak emission angle of light emittingdevice 40 is to some degree a larger angle (about 63°) relative to theoptical axis of light emitting element 42. Here, the “peak emissionangle” means an emission angle of light with the highest luminousintensity.

On the other hand, recently, a further reduction in the number of lightemitting elements in a surface light source device and further thinningof a surface light source device (i.e., making the value of theaforementioned H/P smaller) have been demanded. However, only byadjusting the peak emission angle of light emitting device 40 as insurface light source device 10 set forth in PTL 1, a reduction in thenumber of light emitting elements and the thinning of the surface lightsource device cannot be sufficiently realized. For example, whenadopting light emitting device 40 having the light distributioncharacteristics illustrated in FIG. 2 in surface light source device 10having an H/P illustrated in FIG. 1B of about 0.77, the luminanceunevenness on the light emitting surface may be smaller. However,according to the calculation of the present inventors, when adoptingthis light emitting device 40 in a surface light source device having anH/P of 0.2 or less, light emitted at the peak emission angle fromcertain light emitting device 40 reaches an area between that lightemitting device 40 and adjacent light emitting device 40, of the lowersurface of light diffusion plate 50, and light emitted from that lightemitting device 40 in an angular range from 0° to the peak emissionangle (most part of light emitted from light emitting device 40) failsto reach farther. Therefore, an area of the light emitting surface inthe vicinity of light emitting device 40 becomes relatively brighter,causing luminance unevenness to occur (see FIG. 13). This is caused bythe non-optimized light distribution characteristics of light emittingdevice 40 from 0° to the peak emission angle.

Thus, the conventional direct surface light source device results inhaving increased luminance unevenness on a light emitting surface whenH/P is set to 0.2 or less. That is, in the conventional direct surfacelight source device, H/P cannot be set to 0.2 or less.

An object of the present invention is to provide a direct surface lightsource device with a light emitting element as a light source, having anH/P of 0.2 or less and having less luminance unevenness. Further,another object of the present invention is to provide a displayapparatus having the surface light source device.

Solution to Problem

In order to achieve the aforementioned objects, a surface light sourcedevice of the present invention includes: a substrate; a plurality oflight emitting devices, each of which includes a light emitting elementand a light flux controlling member that controls a distribution ofemission light from the light emitting element, the light emittingdevices being disposed at constant intervals on the substrate; and alight diffusion plate disposed substantially parallel to the substrateover the plurality of light emitting devices, the light diffusion platebeing configured to transmit light from the light emitting device whilediffusing the light, wherein, in an angular range from a direction alongan optical axis of the light emitting device to a direction of emissionof light with the highest luminous intensity from the light emittingdevice, a luminous intensity of light from the light emitting device isgradually increased as an angle of the light relative to the opticalaxis becomes larger, an angle of the light with the highest luminousintensity from the light emitting device is more than 78.7° relative tothe optical axis, and the following Equations 1 and 3 are satisfied.

$\begin{matrix}{\frac{H}{P} \leq 0.2} & \left( {{Equation}\mspace{14mu} 1} \right) \\{\frac{I_{1\text{/}2}}{I_{0}} > 6} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

where P is a center-to-center distance between the light emittingdevices adjacent to each other, H is an interval between the uppersurface of the substrate and the lower surface of the light diffusionplate, I₀ is a luminous intensity of light emitted in the optical axisdirection from the light emitting device, and I_(1/2) is a luminousintensity of light emitted from the light emitting device toward thepoint of a distance P/2 from the intersection point of the optical axisand the lower surface of the light diffusion plate, on the lower surfaceof the light diffusion plate.

A display apparatus of the present invention has a surface light sourcedevice of the present invention, and a member to be irradiated withlight from the surface light source device.

Advantageous Effects of Invention

According to the present invention, there can be provided a thin surfacelight source device and a display apparatus with less energyconsumption.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are drawings illustrating the configuration of a surfacelight source device (surface light emitting unit) set forth in PTL 1;

FIG. 2 is a graph illustrating the light distribution characteristics ofa light emitting device (light emitting unit) set forth in PTL 1;

FIGS. 3A and 3B are drawings illustrating the configuration of a surfacelight source device according to an embodiment;

FIGS. 4A and 4B are sectional views illustrating the configuration of asurface light source device according to the embodiment;

FIG. 5 is a partially enlarged sectional view in which a part of FIG. 4Bis enlarged;

FIGS. 6A to 6E are drawings illustrating the configuration of a lightflux controlling member according to an embodiment;

FIG. 7 is a partially enlarged sectional view of a surface light sourcedevice according to the embodiment for explaining Equations 1, 2, 3 and4;

FIGS. 8A and 8B are graphs illustrating the light distributioncharacteristics of four types of light emitting devices;

FIG. 9 is a graph illustrating the values of H/P and L/P of four typesof surface light source devices;

FIG. 10A is a graph illustrating the values of I_(1/2)/I₀ of four typesof surface light source devices;

FIG. 10B is a graph illustrating the values of I_(1/4)/I₀ of four typesof surface light source devices;

FIGS. 11A and 11B are graphs illustrating the luminance distributions ofa light emitting surface when only one light emitting device is lightedin four types of surface light source devices;

FIG. 12A is a drawing illustrating the luminance distribution of a lightemitting surface of a surface light source device not having a lightflux controlling member;

FIG. 12B is a drawing illustrating the luminance distribution of a lightemitting surface of a surface light source device according to theembodiment of the present invention (H/P≦0.2, L/P>1, I_(1/2)/I₀>6,I_(1/4)/I₀≦2.4);

FIGS. 12C to 12E are drawings illustrating the luminance distribution ofa light emitting surface of a surface light source device of acomparative example (H/P≦0.2, L/P≦1, I_(1/2)/I₀≦6, I_(1/4)/I₀≦2.4);

FIG. 13 is a sectional view illustrating an optical path in a surfacelight source device of the comparative example (H/P≦0.2, L/P≦1,I_(1/2)/I₀>6, I_(1/4)/I₀≦2.4);

FIG. 14A is a graph illustrating the light distribution characteristicsof a light emitting device to be used in a surface light source deviceof an embodiment of the present invention (I_(1/2)/I₀>6, I_(1/4)/I₀≦2.4)and the light distribution characteristics of a light emitting device tobe used in a surface light source of a comparative example(I_(1/2)/I₀≦6, I_(1/4)/I₀≦2.4);

FIG. 14B is a graph illustrating the luminance distribution of a lightemitting surface when only one light emitting device is lighted in thesurface light source device of the embodiment of the present invention(H/P≦0.2, L/P>1, I_(1/2)/I₀>6, I_(1/4)/I₀≦2.4) and in the surface lightsource device of the comparative example (H/P≦0.2, L/P>1, I_(1/2)/I₀≦6,I_(1/4)/I₀≦2.4);

FIG. 15 is a drawing illustrating the luminance distribution of a lightemitting surface of a surface light source device of the comparativeexample (H/P≦0.2, L/P>1, I_(1/2)/I₀≦6, I_(1/4)/I₀≦2.4);

FIGS. 16A and 16B are graphs illustrating the luminance distribution ofa light emitting surface when only one light emitting device is lightedin a surface light source device having light emitting devices, each ofwhich has different values of I_(1/4)/I₀; and

FIG. 17 is a graph illustrating the relationship between I_(1/4)/I₀ andthe luminance of an area in the vicinity of a light emitting device.

DESCRIPTION OF EMBODIMENT

In the following, an embodiment of the present invention will bedescribed in detail with reference to the accompanying drawings. Here,as a typical example of the surface light source device of the presentinvention, a surface light source device suitable as a backlight of aliquid crystal display apparatus will be described. When used with amember (for example, liquid crystal panel) to be irradiated with lightfrom the surface light source device, the surface light source devicecan be used as a display apparatus.

[Configurations of Surface Light Source Device and Light EmittingDevice]

FIGS. 3A and 3B to 5 are drawings illustrating the configuration ofsurface light source device 100 according to an embodiment of thepresent invention. FIG. 3A is a plan view, and FIG. 3B is a front view.FIG. 4A is a sectional view taken along line A-A illustrated in FIG. 3B,and FIG. 4B is a sectional view taken along line B-B illustrated in FIG.3A and line C-C illustrated in FIG. 4A. FIG. 5 is a partially enlargedsectional view in which a part of FIG. 4B is enlarged.

As illustrated in FIGS. 3A and 3B and 4A and 4B, surface light sourcedevice 100 of the present embodiment has housing 110, substrate 120, aplurality of light emitting devices 130 and light diffusion plate 160.Substrate 120 is disposed on the bottom plate of housing 110, and theplurality of light emitting devices 130 are disposed at constantintervals on substrate 120. An opening is provided at the top plate ofhousing 110. Light diffusion plate 160 is disposed substantiallyparallel to substrate 120 over the plurality of light emitting devices130 so as to cover the opening, and functions as a light emittingsurface. While the size of the light emitting surface is notparticularly limited, the size is for example about 400 mm×about 700 mm.The center-to-center distance (pitch) between light emitting devices 130is P (mm), and the interval (height) between the upper surface ofsubstrate 120 and the lower surface of light diffusion plate 160 is H(mm) (see FIG. 7). Surface light source device 100 according to thepresent embodiment satisfies the following Equation 1.

$\begin{matrix}{\frac{H}{P} \leq 0.2} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

As illustrated in FIG. 5, each of the plurality of light emittingdevices 130 is fixed on substrate 120. Each of the plurality of lightemitting devices 130 has light emitting element 140 and light fluxcontrolling member 150.

Light emitting element 140 is a light source of surface light sourcedevice 100. Light emitting element 140 is a light-emitting diode (LED)such as a white light-emitting diode, for example.

Light flux controlling member 150 controls the distribution of lightemitted from light emitting element 140. Light flux controlling member150 is disposed over light emitting element 140 such that its centralaxis coincides with the optical axis of light emitting element 140.Here, the “optical axis of light emitting element” means the centrallight beam of a stereoscopic emission light flux from light emittingelement 140. Optical axis LA of light emitting device 130 coincides withthe optical axis of light emitting element 140 and the central axis oflight flux controlling member 150 (see FIG. 5). Between substrate 120and light flux controlling member 150, there is defined a gap forreleasing heat emitted from light emitting element 140 to the outside.

Light flux controlling member 150 is formed by integral molding. Thematerial for light flux controlling member 150 is not particularlylimited as long as the light of a desired wavelength can pass throughthe material. The material for light flux controlling member 150 is, forexample, a light transmissive resin such as polymethylmethacrylate(PMMA), polycarbonate (PC) or epoxy resin (EP), or glass. The shape oflight flux controlling member 150 will be separately described indetail.

Light diffusion plate 160 is a plate-like member having a lightdiffusing property, and transmits emission light from light emittingdevice 130 while diffusing it. Typically, the size of light diffusionplate 160 is substantially the same as that of a member to beirradiated, such as a liquid crystal panel. For example, light diffusionplate 160 is formed of light transmissive resins such aspolymethylmethacrylate (PMMA), polycarbonate (PC), polystyrene (PS), orstyrene methyl methacrylate copolymerization resin (MS). In order toprovide a light diffusing property, fine irregularities are formed onthe surface of light diffusion plate 160, or light diffusers such asbeads are dispersed inside light diffusion plate 160.

Light from light emitting element 140 is expanded in radial directions(surface directions of substrate 120) by light flux controlling member150. Light emitted from light emitting device 130 reaches lightdiffusion plate 160. Light having reached light diffusion plate 160 istransmitted through light diffusion plate 160 while being diffused.

[Configuration of Light Flux Controlling Member]

FIGS. 6A to 6E are drawings illustrating the configuration of light fluxcontrolling member 150 according to the present embodiment. FIG. 6A is aplan view of light flux controlling member 150, FIG. 6B is a front viewof light flux controlling member 150, FIG. 6C is a side view of lightflux controlling member 150, and FIG. 6D is a bottom view of light fluxcontrolling member 150. In addition, FIG. 6E is a sectional view takenalong line D-D illustrated in FIG. 6A.

As illustrated in FIGS. 6A to 6E, light flux controlling member 150 hasincidence surface 151, rear surface 152, emission surface 153, flange154, and a plurality of legs 155.

Incidence surface 151 is a recessed internal surface formed so as toface light emitting element 140 at the central portion of the lower side(light emitting element 140 side) of light flux controlling member 150.The shape of incidence surface 151 is such a recessed shape that a partof spheroid surface is cut off. Incidence surface 151 allows lightemitted from light emitting element 140 to enter light flux controllingmember 150 while controlling its traveling direction.

Rear surface 152 is a plane positioned at the lower side (light emittingelement 140 side) of light flux controlling member 150 and extending inthe radial direction from the opening edge of the recess.

Emission surface 153 emits light having entered light flux controllingmember 150 toward the outside while controlling its traveling direction.Emission surface 153 is a rotational symmetry plane around the centralaxis of light flux controlling member 150, and is protruded upward(light diffusion plate 160 side) beyond flange 154.

According to the present embodiment, emission surface 153 has firstemission surface 153 a positioned within a predetermined range aroundthe central axis of light flux controlling member 150, second emissionsurface 153 b formed continuously around first emission surface 153 a,and a third emission surface 153 c connecting second emission surface153 b and flange 154 (see FIG. 6E). First emission surface 153 a is asmooth curved surface (recessed curved surface) being convex downward(light emitting element 140 side) disposed at a position intersectingthe central axis (optical axis LA of light emitting device 130) of lightflux controlling member 150. The shape of first emission surface 153 ais such a recessed shape that a part of spherical surface is cut off.Second emission surface 153 b is a smooth curved surface (convex curvedsurface) being convex upward (light diffusion plate 160 side) positionedaround first emission surface 153 a. The shape of second emissionsurface 153 b is such a convex shape that a part of toric surface is cutoff. Third emission surface 153 c is a curved surface positioned aroundsecond emission surface 153 b. In the sectional view as illustrated inFIG. 6E, the section of third emission surface 153 c may be eitherlinear or curved.

Flange 154 is positioned between the outer peripheral portion ofemission surface 153 and the outer peripheral portion of rear surface152, and is protruded outward in the radial direction. The shape offlange 154 is substantially toric. Flange 154 is not always necessary,but, by providing flange 154, it becomes easier to handle and alignlight flux controlling member 150. The thickness of flange 154 is notparticularly limited, and is determined taking account of an arearequired for emission surface 153, the molding property of flange 154,or the like.

The plurality of legs 155 are cylindrical members protruding downward(light emitting element 140 side) from rear surface 152 at the outerperipheral portion of rear surface 152. The plurality of legs 155support light flux controlling member 150 such that it is positionedproperly relative to light emitting element 140.

Light from light emitting element 140 enters light flux controllingmember 150 through incidence surface 151. At that time, light from lightemitting element 140 is expanded in radial directions (directionsorthogonal to the optical axis of light emitting element 140) due to theshape of incidence surface 151. Light having entered light fluxcontrolling member 150 is emitted toward the outside through emissionsurface 153. Also at that time, light from light emitting element 140 isfurther expanded in radial directions (directions orthogonal to theoptical axis of light emitting element 140) due to the shape of emissionsurface 153. As a result, light is emitted in a larger angular range(from 0° to nearly 90° relative to the optical axis) from light emittingdevice 130 (see FIG. 8A). In the angular range (range from 0° to thepeak emission angle) from the direction along optical axis LA of lightemitting device 130 to the direction of emission of light with thehighest luminous intensity from light emitting device 130, the luminousintensity of light from light emitting device 130 is gradually increasedas an angle relative to optical axis LA becomes larger (see FIG. 8A).

FIG. 7 is a partially enlarged sectional view of surface light sourcedevice 100 according to the present embodiment. In this drawing, housing110 is omitted. As described above, surface light source device 100according to the present embodiment satisfies the following Equation 1.

$\begin{matrix}{\frac{H}{P} \leq 0.2} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where P is a center-to-center distance (pitch) between the lightemitting devices 130 adjacent to each other, and H is an interval(height) between the upper surface of substrate 120 and the lowersurface of light diffusion plate 160.

Further, surface light source device 100 satisfies the followingEquation 2 in order to prevent the occurrence of luminance unevenness onthe light emitting surface, while satisfying the aforementionedEquation 1. That is, the shapes of incidence surface 151 and emissionsurface 153 of light flux controlling member 150 are adjusted so as tosatisfy the following Equation 2. As illustrated in FIG. 7, Equation 2means that light emitted at the peak emission angle from certain lightemitting device 130 reaches farther than adjacent light emitting device130. Thereby, it becomes possible to reduce the occurrence of a brightpart (area having a relatively higher luminance) in an area betweenlight emitting devices 130 of the light emitting surface. Since Equation2 is satisfied, the emission angle of light with the highest luminousintensity (peak emission angle), of an emission angle of light emittedfrom light emitting device 130 (optical axis direction: 0°, substratesurface direction: 90°), is more than 78.7°.

$\begin{matrix}{\frac{L}{P} > 1} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where P is a center-to-center distance (pitch) between the lightemitting devices 130 adjacent to each other, and L is a distance fromthe intersection point of optical axis LA of light emitting device 130and the lower surface of light diffusion plate 160 to the point wherelight emitted at the peak emission angle from light emitting device 130reaches the lower surface of light diffusion plate 160.

Further, surface light source device 100 also satisfies the followingEquation 3 in order to prevent the occurrence of luminance unevenness,while satisfying the aforementioned Equation 1. That is, the shapes ofincidence surface 151 and emission surface 153 of light flux controllingmember 150 are adjusted so as to satisfy the following Equation 3. Asillustrated in FIG. 7, Equation 3 means that the luminous intensity(I_(1/2)) of light propagating toward the intermediate point between twolight emitting devices 130 on the lower surface of light diffusion plate160 is six times higher than the luminous intensity (I₀) of lightpropagating toward directly above light emitting device 130. Thereby, itbecomes possible to reduce the occurrence of a dark part (area having arelatively lower luminance) in an area between light emitting devices130 of the light emitting surface.

$\begin{matrix}{\frac{I_{1\text{/}2}}{I_{0}} > 6} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

where I₀ is a luminous intensity of light emitted in the direction ofoptical axis LA from light emitting device 130, and I_(1/2) is aluminous intensity of light emitted from light emitting device 130toward the point of a distance P/2 from the intersection point ofoptical axis LA and the lower surface of light diffusion plate 160, onthe lower surface of light diffusion plate 160.

Further, preferably, surface light source device 100 also satisfies thefollowing Equation 4. That is, it is preferable that the shapes ofincidence surface 151 and emission surface 153 of light flux controllingmember 150 are adjusted so as to satisfy the following Equation 4. Asillustrated in FIG. 7, Equation 4 means that the luminous intensity(I_(1/4)) of light propagating toward the intermediate point (point ofP/4) between the intermediate point between two light emitting devices130 and one light emitting device 130 on the lower surface of lightdiffusion plate 160 is 2.4 times as high as or lower than 2.4 times ofthe luminous intensity (I₀) of light propagating toward directly abovethat light emitting device 130. Thereby, it becomes possible to reducethe occurrence of a bright part (area having a relatively higherluminance) in an area in the vicinity of light emitting device 130 ofthe light emitting surface, allowing the luminance distribution on thelight emitting surface to be more uniform.

$\begin{matrix}{\frac{I_{1\text{/}4}}{I_{0}} \leq 2.4} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

where I₀ is a luminous intensity of light emitted in the direction ofoptical axis LA from light emitting device 130 (I₀≠0), and I_(1/4) is aluminous intensity of light emitted from light emitting device 130toward the point of a distance P/4 from the intersection point ofoptical axis LA and the lower surface of light diffusion plate 160, onthe lower surface of light diffusion plate 160.

[Light Distribution Characteristics of Light Emitting Device andLuminance Distribution of Surface Light Source Device]

The light distribution characteristics of light emitting device 130 tobe used in surface light source device 100 according to the presentembodiment were measured. In addition, for comparison, lightdistribution characteristics were measured also for conventional lightemitting devices to be used in conventional surface light sourcedevices. Light emitting device 130 according to the present embodimentdiffers from the conventional light emitting device only in the shape ofan emission surface of the light flux controlling member. Thecharacteristics of light emitting device 130 according to the presentembodiment and three types of conventional light emitting devices areshown in Table 1.

TABLE 1 Segment Example Comparative Example Name P110 P60 P75 P90Optimum Pitch 110 mm 60 mm 75 mm 90 mm Peak Emission Angle 81° 72° 75°77° Luminous Intensity at 59 cd 38 cd 43 cd 53 cd Peak Emission AngleLuminous Intensity at 2.5 cd 3.3 cd 2.8 cd 2.9 cd 0° (I₀)

FIGS. 8A and 8B are graphs illustrating the light distributioncharacteristics of four types of light emitting devices (P110, P60, P75and P90) shown in Table 1. The abscissa indicates an angle when thecenter of the light emitting surface of the light emitting element isdefined as the origin and optical axis LA of the light emitting deviceis set to 0°. The ordinate indicates a luminous intensity (FIG. 8A) or arelative luminous intensity (FIG. 8B) at each angle. FIG. 8B indicates arelative luminous intensity when the luminous intensity at 0° is set to1 in each light emitting device. The results for light emitting device130 (P110) according to the present embodiment are indicated by a thicksolid line. On the other hand, the measurement results for lightemitting devices (P60, P75 and P90) of the comparative examples areindicated by a thin broken line, a thin solid line or a thin dashedline.

From these graphs, it can be observed that light emitting device 130(P110) of the present embodiment has a peak emission angle of 78.7° ormore, and can generate more amount of light that propagates toward adistant area compared to the light emitting devices (P60, P75 and P90)of the comparative examples.

Next, the luminance distribution of surface light source device 100having light emitting device 130 (P110) of the present embodiment wasmeasured. In addition, for comparison, luminance distributions weremeasured also for surface light source devices having the aforementionedlight emitting devices (P60, P75 and P90) of the comparative examples.The light emitting devices (P10, P60, P75 and P90) were disposed at therespective optimum pitches (see Table 1) inside the surface light sourcedevices having a height H of 19 mm.

FIG. 9 is a graph illustrating values of H/P and L/P of each of thesurface light source devices. As illustrated in this graph, in surfacelight source device 100 according to the present embodiment, H/P is 0.2or less, and L/P is more than 1. That is, surface light source device100 according to the present embodiment satisfies the aforementionedEquations 1 and 2. On the other hand, surface light source deviceshaving the light emitting devices (P60, P75 and P90) of the comparativeexamples have an H/P of more than 0.2, and an L/P of 1 or less. That is,neither of these surface light source devices satisfies theaforementioned Equations 1 and 2.

FIG. 10A is a graph illustrating the value of I_(1/2)/I₀ of each of thesurface light source devices, and FIG. 10B is a graph illustrating thevalue of I_(1/4)/I₀ of each of the surface light source devices. Asillustrated in these graphs, in surface light source device 100according to the present embodiment, I_(1/2)/I₀ is more than 6, andI_(1/4)/I₀ is 2.4 or less. That is, surface light source device 100according to the present embodiment satisfies the aforementionedEquations 3 and 4. On the other hand, in surface light source deviceshaving the light emitting devices (P60, P75 and P90) of the comparativeexamples, I_(1/4)/I₀ is 2.4 or less, while I_(1/2)/I₀ is 6 or less. Thatis, these surface light source devices satisfy the aforementionedEquation 4, but do not satisfy the aforementioned Equation 3.

FIGS. 11A and 11B are graphs illustrating luminance distributions of thelight emitting surface when only one light emitting device is lighted ineach of the surface light source devices. The abscissa indicates adistance from optical axis LA of the light emitting device. The ordinateindicates a luminance (FIG. 11A) or a relative luminance (FIG. 11B) ateach point. FIG. 11B indicates a relative luminance when the luminanceon optical axis LA is set to 1 in each of the surface light sourcedevices. The results for the surface light source device having lightemitting device 130 (P110) according to the present embodiment areindicated by a thick solid line. On the other hand, the measurementresults for the surface light source devices having light emittingdevices (P60, P75 and P90) of the comparative examples are indicated bya thin broken line, a thin solid line or a thin dashed line.

From the graph illustrated in FIG. 11B, it can be observed that, whenlight emitting devices 130 (P110) are disposed at 110 mm pitch(H/P=0.17), sufficient brightness is obtained at the intermediateposition (±55 mm) between light emitting devices 130 on the lightemitting surface, whereas, when the light emitting devices (P60 and P75)of the comparative examples are disposed at 110 mm pitch (H/P=0.17),brightness is insufficient at the intermediate position (±55 mm)Further, when the light emitting devices are disposed in a matrix mannerat 110 mm pitch (H/P=0.17), the center-to-center distance between lightemitting devices in a diagonal direction is about 155 mm. When the lightemitting devices (P90) of the comparative example are disposed at 110 mmpitch (H/P=0.17), brightness is insufficient at the intermediateposition (±77.5 mm) in a diagonal direction on the light emittingsurface. In contrast thereto, it can be observed that, in light emittingdevice 130 according to the present embodiment, sufficient luminance canbe obtained even in the tails of the luminance distribution on the lightemitting surface.

FIGS. 12A to 12D are luminance distributions of the light emittingsurface when sixteen light emitting devices are lighted in each of thesurface light source devices. FIG. 12A is a graph illustrating aluminance distribution of the light emitting surface when the light fluxcontrolling member is removed, FIG. 12B is a luminance distribution ofthe light emitting surface of surface light source device 100 accordingto the present embodiment, FIG. 12C is a luminance distribution of thelight emitting surface of the surface light source device having thelight emitting device (P60) of the comparative example, FIG. 12D is aluminance distribution of the light emitting surface of the surfacelight source device having the light emitting device (P75) of thecomparative example, and FIG. 12E is a luminance distribution of thelight emitting surface of the surface light source device having thelight emitting device (P90) of the comparative example. The respectivelight emitting devices are disposed at the pitch of 110 mm inside thesurface light source device having a height H of 19 mm, and H/P is 0.17in any of the surface light source devices.

As illustrated in FIGS. 12C to 12E, in the surface light source devices(H/P≦0.2, L/P≦1, I_(1/2)/I₀≦6, I_(1/4)/I₀≦2.4) having the light fluxcontrolling member of the comparative example, luminance unevenness waslarger. In contrast thereto, in surface light source device 100(H/P≦0.2, L/P>1, I_(1/2)/I₀>6, I_(1/4)/I₀≦2.4) according to the presentembodiment, luminance unevenness was smaller, despite the fact that H/Pwas 0.2 or less as illustrated in FIG. 12B. Here, the phrase “luminanceunevenness was smaller” means that the ratio of the minimum luminancerelative to the maximum luminance in an area between light emittingdevices of the light emitting surface is 95% or more.

As has been described, surface light source device 100 according to thepresent embodiment can emit uniform light from the light emittingsurface despite the fact that H/P is 0.2 or less.

It is noted that, when the surface light source device does not satisfythe aforementioned Equation 2, light emitted at a peak emission angle(e.g., 63°) from the light emitting device reaches an area between lightemitting devices, of the lower surface of the light diffusion plate.Therefore, in surface light source device 10′ (H/P≦0.2, L/P≦1,I_(1/2)/I₀>6, I_(1/4)/I₀≦2.4) not satisfying only the aforementionedEquation 2, as illustrated in FIG. 13, most of emission light from lightemitting device 40 reaches an area in the vicinity of light emittingdevice 40 (area where light emitted at the peak emission angle reaches)of the light emitting surface, thereby causing the occurrence of an areawith insufficient amount of light between light emitting devices 40 onthe light emitting surface. As a result, bright part B that isrelatively brighter is formed in an area in the vicinity of lightemitting device 40 on the light emitting surface, resulting in luminanceunevenness occurring (cf. FIG. 1B).

Further, when the surface light source device does not satisfy theaforementioned Equation 3, the light distribution characteristics of thelight emitting device are as indicated by a broken line in FIG. 14A. InFIG. 14A, a solid line is a curve showing the light distributioncharacteristics of light emitting device 130 (P110) according to thepresent embodiment. FIG. 14B is a graph illustrating the luminancedistribution of the light emitting surface when only one light emittingdevice is lighted in a surface light source device having the lightemitting device. In FIG. 14B, a broken line is a curve showing theluminance distribution of the light emitting surface of the surfacelight source device (H/P≦0.2, L/P>1, I_(1/2)/I₀≦6, I_(1/4)/I₀≦2.4) notsatisfying only the aforementioned Equation 3, and a solid line is acurve showing the luminance distribution of the light emitting surfaceof surface light source device 100 (H/P≦0.2, L/P>1, I_(1/2)/I₀>6,I_(1/4)/I₀≦2.4) according to the present embodiment. Further, FIG. 15 isa luminance distribution of the light emitting surface when sixteenlight emitting devices are lighted in the surface light source device(H/P≦0.2, L/P>1, I_(1/2)/I₀≦6, I_(1/4)/I₀≦2.4) not satisfying only theaforementioned Equation 3. From these results, it can be observed that,when a surface light source device does not satisfy the aforementionedEquation 3, an area between light emitting devices 40 of the lightemitting surface becomes relatively darker, causing luminance unevennessto occur.

Further, even when the surface light source device does not satisfy theaforementioned Equation 4, if the aforementioned Equations 1 to 3 aresatisfied, then luminance unevenness is sufficiently suppressed.However, when the surface light source device also satisfies theaforementioned Equation 4, the luminance distribution on the lightemitting surface becomes more uniform. FIGS. 16A and 16B are graphsillustrating the luminance distribution of the light emitting surfacewhen only one light emitting device is lighted in the surface lightsource device having light emitting devices, each of which has differentvalues of I_(1/4)/I₀. FIG. 16B illustrates the peak portion in the graphof FIG. 16A in an enlarged manner. A solid line is a curve showing theluminance distribution of the light emitting surface of the surfacelight source device (H/P≦0.2, L/P>1, I_(1/2)/I₀>6, I_(1/4)/I₀=1.6)having light emitting device 130 (P110) according to the presentembodiment. A thin broken line, a thin solid line, a thin dashed lineand a thin chain double-dashed line are curves showing the luminancedistributions of the light emitting surface of the surface light sourcedevices (H/P≦0.2, L/P>1, I_(1/2)/I₀>6, I_(1/4)/I₀=2.0, 2.1, 2.2, 2.3,2.4, 2.5) having other light emitting devices. The surface light sourcedevice having light emitting device 130 (P110) according to the presentembodiment and the surface light source devices having other lightemitting devices differ from each other only in the value of I_(1/4)/I₀.From these results, it can be observed that, when the value ofI_(1/4)/I₀ varies, the luminance of an area in the vicinity of lightemitting device 40, of the light emitting surface, varies. From theviewpoint of further equalizing the luminance distribution on the lightemitting surface, the luminance of an area in the vicinity of lightemitting device 40 is preferably lower than the luminance of a pointdirectly above light emitting device 40.

FIG. 17 is a graph illustrating the relationship between I_(1/4)/I₀ andthe luminance of an area in the vicinity of light emitting device 40.The ordinate indicates a luminance at a point 18 mm away from theoptical axis of light emitting device 40 (point where the peak exists inthe graph of FIG. 16A) on the light emitting surface, when the luminanceof a point on the light emitting surface directly above light emittingdevice 40 is set to 1. From this graph, it can be observed that, whenI_(1/4)/I₀ is 2.4 or less, the luminance of an area in the vicinity oflight emitting device 40 becomes lower than the luminance of a pointdirectly above light emitting device 40. Accordingly, from the viewpointof further equalizing the luminance distribution on the light emittingsurface, I_(1/4)/I₀ is preferably 2.4 or less.

INDUSTRIAL APPLICABILITY

The surface light source device according to the present invention isapplicable, for example, to a back light of a liquid crystal displayapparatus, a signboard, or a generally-used illumination apparatus.

REFERENCE SIGNS LIST

-   10, 10′ Surface light source device (Surface light emitting unit)-   20 Housing-   30 Substrate (Mounting plate)-   40 Light emitting device (Light emitting unit)-   42 Light emitting element-   44 Light flux controlling member (Lens part)-   50 Light diffusion plate-   100 Surface light source device-   110 Housing-   120 Substrate-   130 Light emitting device-   140 Light emitting element-   150 Light flux controlling member-   151 Incidence surface-   152 Rear surface-   153 Emission surface-   153 a First emission surface-   153 b Second emission surface-   153 c Third emission surface-   154 Flange-   155 Leg-   160 Light diffusion plate-   LA Optical axis of light emitting device

What is claimed is:
 1. A surface light source device comprising: asubstrate; a plurality of light emitting devices, each of which includesa light emitting element, and a light flux controlling member thatcontrols a distribution of emission light from the light emittingelement, the light emitting devices being disposed at constant intervalson the substrate; and a light diffusion plate disposed substantiallyparallel to the substrate over the plurality of light emitting devices,the light diffusion plate being configured to transmit light from eachlight emitting device while diffusing the light, wherein in an angularrange from a direction along an optical axis of the light emittingdevice to a direction of emission of light with the highest luminousintensity from the light emitting device, a luminous intensity of lightfrom the light emitting device is gradually increased as an angle of thelight relative to the optical axis becomes larger, an angle of the lightwith the highest luminous intensity from the light emitting device ismore than 78.7° relative to the optical axis, and the surface lightsource device is constructed such that following Equations 1 and 3 aresatisfied: $\begin{matrix}{\frac{H}{P} \leq 0.2} & \left( {{Equation}\mspace{14mu} 1} \right) \\{\frac{I_{1\text{/}2}}{I_{0}} > 6} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$ where P is a center-to-center distance between the lightemitting devices adjacent to each other, H is an interval between anupper surface of the substrate and a lower surface of the lightdiffusion plate, I₀ is a luminous intensity of light emitted in theoptical axis direction from the light emitting device, and I_(1/2) is aluminous intensity of light emitted from the light emitting devicetoward a point of a distance P/2 from the intersection point of theoptical axis and the lower surface of the light diffusion plate, on thelower surface of the light diffusion plate.
 2. The surface light sourcedevice according to claim 1, wherein the surface light source device isconstructed such that following Equation 4 is further satisfied:$\begin{matrix}{\frac{I_{1\text{/}4}}{I_{0}} \leq 2.4} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$ where I_(1/4) is a luminous intensity of light emittedfrom the light emitting device toward a point of a distance P/4 from theintersection point of the optical axis and the lower surface of thelight diffusion plate, on the lower surface of the light diffusionplate.
 3. The surface light source device according to claim 1, whereinthe light flux controlling member includes an incidence surface that isa recessed internal surface disposed so as to face the light emittingelement and configured to receive light emitted from the light emittingelement, and an emission surface that is disposed so as to face thelight diffusion plate and configured to emit light having entered theincidence surface with the light being controlled for a travelingdirection of the light, and the emission surface includes a firstemission surface that is a recessed curved surface disposed at aposition intersecting the optical axis, and a second emission surfacethat is a convex curved surface disposed around the first emissionsurface.
 4. The surface light source device according to claim 2,wherein the light flux controlling member includes an incidence surfacethat is a recessed internal surface disposed so as to face the lightemitting element and that light emitted from the light emitting elemententers, and an emission surface that is disposed so as to face the lightdiffusion plate and that emits light having entered the incidencesurface while being controlled for a traveling direction of the light,and the emission surface includes a first emission surface that is arecessed curved surface disposed at a position intersecting the opticalaxis, and a second emission surface that is a convex curved surfacedisposed around the first emission surface.
 5. A display apparatuscomprising: the surface light source device according to claim 1; and amember to be irradiated with light from the surface light source device.6. A display apparatus comprising: the surface light source deviceaccording to claim 2; and a member to be irradiated with light from thesurface light source device.
 7. A display apparatus comprising: thesurface light source device according to claim 3; and a member to beirradiated with light from the surface light source device.
 8. A displayapparatus comprising: the surface light source device according to claim4; and a member to be irradiated with light from the surface lightsource device.